1. Field of the Invention
The present invention relates to a light deflector that deflects an incident light, a method of manufacturing the light deflector, an optical device using the light deflector, and a torsion oscillating member.
2. Related Background Art
Until now, in a mechanical element that requires high-speed operation, its inertia causes a factor that largely impedes a drive speed. In particular, a mechanical element that rotationally oscillates within a given angle is required to reduce an inertia moment. In this case, however, attention is generally paid to a prevention of the rigidity of the driven mechanical element from being deteriorated. For this purpose, there has been widely known a method in which the mechanical element is of a hollow structure, and a method of fixing a reinforcement material (hereinafter referred to as “rib”).
By the way, in recent years, with the development of the microelectronics represented by the high integration of a semiconductor device, a variety of devices, which are high in function and small in sizes are produced. For example, an image display device such as a laser printer or a head mount display, which-conducts optical scanning by using a light deflector, and a light reading device such as an input device including a bar code reader etc. are also added with a high function and are made small in sizes. Those devices are still required to be more downsized. As the light deflector that satisfies the above-mentioned requirements, there has been proposed, for example, a light deflector that scans a light with employing a structure in which a micro mirror is torsion-oscillated by using the micro-machining technique.
The micro mirror used in the light deflector of this type is demanded to have two performances of high-speed drive and high rigidity. In particular, in the case where the rigidity of the micro mirror is short, the micro mirror receives an inertia force due to its weight to cause a mirror surface largely bend at the time of driving. Because the deflection remarkably deteriorates the optical characteristics of the mirror, the performance of the light deflector is lowered. Also, there are many cases in which the generating force of an actuator is restricted in the light reflector of this type, and when the spring rigidity of an elastic support portion is increased for the high-speed drive, there arises such a problem in that the deflection angle is remarkably deteriorated. For that reason, a reduction of the inertia moment of the micro mirror, which is a movable part is required in order to enlarge the light reflection angle.
Under the above circumstances, the following structure has been proposed in order to reduce the inertia moment while keeping the rigidity of the micro mirror. FIGS. 1A and 1B are diagrams showing a light reflector disclosed in “Proceedings of MENS2000,” pp. 704–709. FIG. 1A is a perspective view showing a micro mirror portion, and FIG. 1B is a cross-sectional view taken along the line 1B—1B of FIG. 1A.
A mirror portion 1001 of the light deflector is structured in such a manner that a cylindrical rib 1004 of single crystal silicon is fixed onto a polycrystal silicon film 1003 on which a metal thin film 1002 for reflecting a light from a light source is formed. The mirror portion 1001 is coupled to a support substrate (not shown) through a torsion spring (not shown) of polycrystal silicon that is elastically supported so as to be torsion-oscillatable. The torsion spring is formed on the same plane as the polycrystal silicon film 1003 of the mirror portion 1001. In this light deflector, such a structure is employed in which the torsion spring is formed on the polycrystal silicon film 1003 on which the metal thin film 1002, which is a reflection surface, is formed, and the torsion spring is reinforced by the cylindrical rib 1004. With this structure, because the rigidity greatly increases as compared with a case using a single unit of the polycrystal silicon film 1003 as the mirror portion 1001, a micro mirror that is high in rigidity and low in inertia moment may be obtained.
However, the light deflector disclosed in the above-mentioned literature is implemented by using the cylindrical rib 1004, which is 13 μm in width and 15 μm in thickness, and the polycrystal silicon film 1003, which is 550 μm in diameter and 1.5 μm in thickness. Accordingly, in the conventional light deflector, the dynamic deflection is not sufficiently suppressed due to insufficient rigidity of the polycrystal silicon film 1003. If an attempt is made to further suppress the dynamic deflection without changing the structure of the conventional light deflector, there may be proposed the following two methods, which can be employed while taking manufacturing limits into consideration: a method of increasing the residual stress of the polycrystal silicon film 1003 and a method of increasing the thickness of the cylindrical rib 1004. However, both of those methods suffer from such a problem in that the performance of the light deflector is remarkably deteriorated for the reasons stated below.    1) In the case where the residual stress of the polycrystal silicon film 1003 is increased, the static flatness of the polycrystal silicon film 1003 cannot be obtained. In addition, a reflection surface formed on the polycrystal silicon film 1003 also has a large curve, thereby causing the deformation of the deflected light to deteriorate the reflection performance.    2) In the case where the thickness of the cylindrical rib 1004 is increased, the mass of a portion at which the moment arm is maximum, increases. As a result, the inertia moment greatly increases. Also, because the position of the center of gravity of the entire mirror portion 1001 is eccentric from the center axis of the torsion of the torsion spring (hereinafter referred to as “torsion axis”), unnecessary oscillations are liable to occur, thereby deteriorating the deflection performance.